Purine nucleosides stimulate the phosphorylation of specific proteins in extracts of porcine brain. Inosine and guanosine are most effective. Half maximal stimulation occurs at 15 MuM inosine or guanosine. Phosphorylation occurs with either gamma-(32P) ATP or (32P) Pi. The phosphorylated proteins have Mr's of 63,000 and 29,000. The properties of the phosphoryl bond suggest it is an acyl linkage. Ca and Mg stimulate the rate of the phosphorylation. Calmodulin is not involved in the reaction. Tissue specificity studies demonstrate that the nucleoside stimulated phosphorylation is found in brain, heart, kidney, testis, but not liver.